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How to fix a backflow preventer leak

We'll locate the leak, tighten test cocks, replace O-rings, and tell you when to call a plumber.

How-to · Home plumbing
15–30 min
What you'll need
  • Replacement O-rings (match your device—bring the old one to the store)
  • Wrench set
  • Towel (to dry the device when locating the leak)

At a glance

  • Shut off the water supply on both sides of the backflow preventer before working on it.
  • Dry the device and turn water on briefly to locate the leak—test cocks, body connections, or relief port.
  • Tighten test cock caps with a wrench; replace O-rings if they still leak after tightening.
  • If the relief port drips constantly, the internal check valves may have failed—call a plumber.
  • Some backflow preventers require annual testing by a certified tester; check local codes.
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Steps

Goal: Shut off the water, locate the leak, fix it if possible, and know when to call a plumber.

  • Turn off the valves on both sides of the backflow preventer. Open a downstream faucet to relieve pressure.
  • Good: No water flow. Proceed to Locate the leak.
  • Bad: Cannot shut off the water—call a plumber.

Locate the leak

Goal: Find where the water is escaping—test cocks, body connections, or relief port.

  • Dry the device with a towel. Turn the water back on briefly. Watch for drips at the test cocks (small valves on the body), the body connections, or the relief port (drain opening).
  • Good: You see where the water emerges. Proceed to Tighten test cocks if the leak is at test cocks, or Check the relief port if it is at the relief port.
  • Bad: Cannot locate the leak or it is from the body itself—call a plumber.

Tighten test cocks

Goal: Stop leaks at the test cocks by tightening caps or replacing O-rings.

  • Tighten the test cock caps with a wrench. Do not over-tighten—plastic caps can crack.
  • If they still leak, shut off the water, remove the caps, and replace the O-rings with exact matches.
  • Good: No more drip at the test cocks. Proceed to Verification.
  • Bad: Still leaking after tightening and O-ring replacement—call a plumber.

Check the relief port

Goal: Determine if the relief port drip is temporary (debris) or constant (internal failure).

  • The relief port opens when there is a pressure differential. Flush the system by opening and closing the downstream valve.
  • Good: The drip stops—debris may have cleared. Proceed to Verification.
  • Bad: The relief port drips constantly—internal check valve failure. Call a plumber for repair or replacement.

When to get help

Call a plumber if:

  • The relief port drips constantly.
  • The body is leaking.
  • Tightening and O-ring replacement did not stop the leak.
  • The device needs replacement.

Backflow preventers protect drinking water—repairs must be done correctly. Some areas require certified testers for annual testing; check local codes.

Verification

  • No drips at the test cocks or body connections when the water is on.
  • The relief port does not drip when the system is operating normally.
  • Run the irrigation or downstream fixture for several minutes and confirm the area stays dry.

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a backflow preventer leak?
Common causes: loose test cock caps, worn O-rings, debris holding a check valve open, or a failed internal component. Test cocks are the most common leak point. A constant drip from the relief port suggests internal failure.
Can I fix a backflow preventer leak myself?
You can tighten test cock caps and replace O-rings. Shut off the water first. If the relief port drips constantly or the body is leaking, call a plumber. Some jurisdictions require certified testers for backflow devices.
When should I call a plumber for a backflow preventer?
Call a plumber if: the relief port drips constantly, the body is leaking, tightening did not stop the leak, or the device needs replacement. Backflow preventers protect drinking water—repairs should be done correctly.

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